1. Field of the Invention
The present invention relates to an antenna which can be operated in a number of frequency bands, and which is preferably suitable for use in frequency bands for different Standards of mobile radio networks.
2. Description of the Prior Art
In recent years, mobile radio networks have been developed to different Standards which operate in different frequency bands. For example, the GSM Standard mobile radio network operates in the region around 900 MHz, the PCN Standard mobile radio network operates in the region around 1800 MHz, and the PCS Standard mobile radio network operates in the region around 1900 MHz. In this case, it should be noted that the frequency bands for the PCN and PCS Standards overlap one another.
It is, accordingly, desirable to produce mobile radio telephones or similar devices which can be operated in a number of different frequency bands; that is, which are able to work with different mobile radio network Standards. This requires the mobile radio telephones to have one or more antennas which also need to have different resonant frequencies. The resonant frequencies are, in this case, those for the espective frequency bands of the desired mobile radio networks. The reflection factor at these resonant frequencies must be as low as possible, and an adequate bandwidth also must be available to allow the mobile radio telephone to be operated in the respective frequency bands of the mobile radio networks to the various Standards.
A further major factor for the design of antennas for mobile radio networks is that the dimensions are subject to severe limitations for design reasons.
In previous antenna structures, which include a number of antennas, two helical antennas or other shapes such as meandering structures have been used; for example, for covering two different frequency bands. However, these solution approaches require more space than, for example, a simple helical antenna and/or their performance is poorer.
An antenna structure whose essential structure is illustrated schematically in FIG. 5 is known from EP-A-747990. This antenna structure has a first antenna element 10 and a second antenna element 20. The first antenna element 10 has a helical shape, and the second antenna element 20 is in the form of a straight rod or conductor. The two antenna elements 10 and 20 are connected to one another at a common feedpoint 30, and at least part of the second antenna element 20 is arranged inside the first antenna element 10.
In the antenna structure shown in FIG. 5, the first and second antenna elements 10 and 20, respectively, have different resonant frequencies to one another. Thus, the antenna structure shown in FIG. 5 can be operated in at least two frequency bands; for example, two frequency bands of mobile radio networks.
However, the already described antenna structure has considerable disadvantages. The mechanical design of the antenna structure is complex since the antenna structure includes a first and a second antenna element 10 and 20, respectively, with at least part of the second antenna element 20 being arranged inside the first antenna element 10. For this reason, a large amount of effort has to be accepted to manufacture the antenna structure.
Furthermore, the two antenna elements 10 and 20 are located physically close to one another which can lead to problems, such as a short-circuit. The antenna structure also has a narrow bandwidth in the region of one of the resonant frequencies, which can lead to problems during operation in certain mobile radio networks.
Finally, the antenna structure requires a matching network in order to achieve a matching to, normally, 50 Ω. Such a matching network causes losses in the system, however, owing to the components required for this network.
The present invention has been brought about as a result of the problems with the prior art which have been mentioned above, and its object, accordingly, is to provide an antenna which can be operated in a number of frequency bands, has a simple and low-cost structure and can be produced easily.